Objectives of the proposed research are determination of the relevance to vision, and the precise intracellular localization of vertebrate retinal phosphodiesterase photoreceptor outer segment, but evidence for its native location in that part of the cell is debatable. Methods to be used include: first, following the enzymes during fractionation of intact retinas, and conducting assays to differentiate among them as they appear in association with resulting fractions: second, purifying the enzymes from the fractions, making specific antibodies to them, labeling the antibodies with fluorescein and ferritin, and visualizing the bound antibody in sectioned retinas by light and electron microscopy, light-scattering measurements will be correlated with phosphodiesterase activity in an effort to determine the relationship of the enzymes to ion permeability of the outer segment.